Method for making an infrared sensor

ABSTRACT

An infrared sensitive photoconductive material is produced by growing a ternary compound of the formulation Hg(1 x) Cdx Te from a gaseous mixture of mercury, cadmium and tellurium onto a substrate which promotes polycrystalline growth and is chemically inert vis-a-vis the constituent gases. Suitable substrate materials are quartz, sapphire, and certain types of glass which are nonmeltable at growth temperatures of the ternary compound. The method preferably grows the polycrystalline material from a gaseous mixture of mercury, cadmium and tellurium heated to a temperature which inhibits binary combinations and then is rapidly cooled to supersaturation very close to the surface of a solid amorphous substrate material although crystalline substrates may be used provided the lattice structure in growth is incompatible with the lattice of the ternary compound.

United States Patent Lee et a1.

1 Feb. 15, 1972 S. McDermott, Athens, Pa.; Edward S. Pan, Pcughkeepsie,NY.

[73] Assignee: International Business Machines Corporation, Armonk NY{22] Filed: Nov. 17, 1969 121] Appl. No.: 877.312

[52) U.S.Cl 117/212,117/201,117/217. 117/106 R, 252/623 R [51] lnt.Cl.-H0l|7l36 [58] FieldoiSearch 17/201.106A,106R,2l2, 117/217; 252/623 ZB,6273 ZT, 623 R 70 VACUUN UMP l0 SUBMIT SOURCE 3,433,684 3/1969 Zanowicket a1 ..117/106 A Primary Examiner-William Li Jarvis Att0rne vl-lanifinand Jancin and John S Gasper [5 7} ABSTRACT An infrared sensitivephotoconductive material is produced by growing a ternary compound ofthe formulation Hg n Cd Te from a gaseous mixture of mercury, cadmiumand tellurium onto a substrate which promotes polycrystalline growth andis chemically inert visa-vis the constituent gases, Suitable substratematerials are quartz, sapphire, and certain types of glass which arenonmeltable at growth temperatures ofthe ternary compound The methodpreferably grows the polycrystalline material from a gaseous mixture ofmercury, cadmium and tellurium heated to a temperature which inhibitsbinary combinations and then is rapidly cooled to supersaturation veryclose to the surface of a solid amorphous substrate material althoughcrystalline substrates may be used provided the lattice structure ingrowth is incompatible with the lattice of the ternary compound,

11 C1ain1s,4 Drawing Figures PATENTEBFEB 15 ran 3.842.529

sum 1 or 2 T0 COOLANT SOURCE INVENTORS ROBERT E. LEE

PHILIP S. MCDERMOTT EDWARD S. P

ATTORNEY PAIENIEMmsmz 3.642.529

SHEEI 2 or 2 ,19 TEMP. 11 REGULATOR r -w 16 82 l l HI I BLACK AMP WAVE622M i i BIAS ANALYZER l I L. J COOLING CHAMBER METHOD FOR MAKING ANINFRARED SENSOR CROSS-REFERENCES TO RELATED APPLICATIONS BAC KGROU ND OFTHE INVENTION 1. Field of the Invention This invention relates tosemiconductor devices in the forms of ternary compounds and particularlyto a ternary semiconductor material which is infrared sensitive and itsmethod of production.

2. Description of the Prior Art It is well known in the prior art toproduce ternary compounds from the Il-Vl valence groups which areinfrared sensitive. Heretofore it was long thought that such materialsto be radiation sensitive in the IR range and to operate satisfactorilyas semiconductor IR detector devices would have to be monocrystalline instructure. The use and manufacture of the monocrystalline sensormaterial presented substantial problems. For epitaxially producedmaterial, for example, the growth substrate had to be carefully selectedso that its lattice structure at the growth temperature of the compoundwas compatible with the lattice of the grown film. This narrowed thechoice to one material in most cases and presented other limitations onusing variation in temperature as a technique for controllingconstituent formulations of the compound. Further, the growth size ofthe monocrystals tended to be limited because of limited substrate sizeand shape.

SUMMARY OF THE INVENTION The broad object of the present invention is toprovide an improved sensor device and method of manufacture.

It is a specific object to provide a sensor device and process ofmanufacture which overcomes the above-mentioned limitations associatedwith monocrystalline sensor materials.

In accordance with this invention, an infrared detector device isprovided in which polycrystalline material is used. It was discoveredthat polycrystalline ternary compounds having the formulation Hg Cd,Tewhere x is greater than zero and less than one is infrared sensitive. Itwas further discovered that devices using such material can be madewhich approach the theoretical limit of sensitivity for specifiedwavelengths. In the preferred embodiment, the invention is practiced bygrowing a ternary compound of mercury, cadmium, tellurium by rapidlycooling to supersaturation for growth onto a solid substrate which. ingeneral, promotes polycrystalline growth. Specifically. the gaseousmixture is supersaturated and grown on an amorphous substrate such asquartz or a glass which will remain solid and is inert relative to thereactant gases. A particular glass is a heat resistant glass of the typesold commercially under the trade name of Vycor. Polycrystalline materials have also been grown on single crystal quartz, sapphire (A1 andalumina. Other materials may also be used for the substrate whichsatisfy the general criteria.

It will be seen from this discovery that greater freedom of choice isobtained in the selection of a growth substrate. Because of the greaterability to deposit on a greater variety of substrates it becomes morereadily possible to integrate infrared sensor material with othersemiconductor materials to provide monolithic detector structures.Growth over a wider surface area becomes possible within the limits ofthe growth equipment thereby producing a greater yield in the productionof the sensor material. Further, sensor devices using polycrystallinemercury, cadmium, telluride compounds have shown good high-frequencyresponse characteristics.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments ofthe invention, as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a cross-sectional view andschematic of an apparatus useful for practicing the present invention;

FIG. 2 is a plan view ofa sensor device of the present invention;

FIG. 3 is a side elevation of the sensor device of FIG. 2; and

FIG. 4 is a schematic ofa sensor device in combination with anelectrical circuit device for sensing infrared radiation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a vaporgrowing apparatus for practicing this invention comprises sourcefurnaces I0. I]. and 12 connected in parallel between a source I3 of aninert carrier gas and a mixing furnace 14. which in turn is connected toa reaction furnace 15 connected to apparatus 16 for venting the inertgas to the atmosphere. Source furnace 10 comprises a quartz chamber 17wound with a heating coil I8. A supply of elemental cadmium 19,preferably in the form of pellets in a quartz boat, is located withinthe heating zone established by coil 18. A current supply and regulatingmeans of suitable type (not shown) which is independently operable, isconnected to heating coil 18 to maintain temperature levels to effectvolatilizing of the elemental cadmium into the hydrogen gas stream as itflows through chamber 17. The channel for supplying hydrogen gas tochamber 17 comprises tube 20 connected to chamber 17 via airtight seal21, and through flow meter 22 and flow valve 23 to a common flow line24.

Source furnace 11 comprises a quartz chamber 25 wound with heating coil26 electrically connected and controlled in essentially the same manneras coil 18 of furnace I0. A supply 27 of elemental teilurium, preferablyin pellet form ina quartz boat, or the like, is located within theheating zone of coil 26 to be volatilized and added to a hydrogen gasstream flowing through chamber 25. The carrier gas channel to furnace llcomprises a tube 28 connected by an airtight seal 29 to chamber 25 andthrough flow meter 30 and flow valve 3] to supply line 24.

Source furnace 12 comprises a T-shaped quartz tube 32 having one branchconnected by an airtight seal 33 to tube 34 and through flow meter 35and valve 36 to supply line 24. The second branch of chamber 32 isconnected to a mercury supply well 37. Liquid mercury 38 is fed bygravity from an external reservoir 39 through connecting tube 40 to well37. An electrical coil 4] which is wound entirely around the well 37 aswell as the entire junction area of tube 32 is electricallyconnected toa current source and regulating means of a suita ble type for vaporizingthe mercury at predetermined temperature and pressure levels foraddition to a hydrogen gas stream flowing through tube 32. Theconnection of tube 32 to well 37 is preferably made long and the windingof coil 4| is such as to allow a measure of preheating of the mercuryvapors prior to their addition to the hydrogen gas stream in tube 32.The flow of hydrogen gas from source 13 is suitably measured forregulation means such as bubble column 42. or the like, connected tosupply line 24.

As shown, the mixing furnace 14 comprises a cylindrical quartz chamber43 entirely wound with a heating coil 44 which is electrically connectedto suitable current source and regulating means (not shown) which may beindependently operable to maintain temperature of the mixing chamber 43at levels to assure proper constituent control. The constituent gasesmixed with the hydrogen carrier flow from furnaces 10, 11, and 12 intochamber 43 where mixing is produced by a series of baffles 4649. Furtherdetails of construction of the mixing furnace chamber 43 may beunderstood by reference to the Manley et al., application mentionedsupra.

The reaction furnace 15 comprises a cylindrical quartz reaction chamber50, a pair of coaxial heating coils SI and S2 wound thereon, and a meansfor supporting a growth substrate 58 at a selectable growth siteposition within the chamber relative to the heating coils. The reactionchamber 50 is preferably designed with a removable cylindrical section53 which is provided with a central opening and joins with the rest ofthe reaction chamber at airtight seal 54. The substrate supportcomprises a cylindrical pedestal tube 56 which is in serted through thecentral opening of bottom section 53. Sealing means, such as O-rings 56,are provided between pedestal tube 56 and chamber section 53. A growthsubstrate 58 is at tached by suitable means such as spring clip 59.Cooling means comprises a silver heat sink cylinder 60 inserted withinpedestal tube 56, and tube 6], connected through flow meter 62 to an aircoolant source. Both the heat sink 60 and spring clip 59 structures maybe other than the type shown in the above-mentioned copendingapplication. A viewing port 63 is provided in reaction chamber 50 in thegeneral area of the desired growth site. Venting of the carrier gas fromthe system is provided by tube 64 connected through valve 65 to a coldtrap 66. if the carrier gas is to be burned when vented, as in the casewhere hydrogen is used, an ignition device, such as coil 67, may beused. The system is also connected to a vacuum pump from tube 64 throughtube 68 and valve 69.

As discussed in greater detail in said Manley et at, application, theheating coils 51 and 52 are connected to separate current source andregulator means, and are relatively movable longitudinally alongreaction chamber 50 to provide gap 70 as a means of regulating thermalgradients.

The method for operating the apparatus of FIG. 1 is explained insubstantial detail in the said Manley et al. applica tion. Generally,the same procedures are followed in practicing this invention. However,the distinguishing feature of the present invention involves the use ofsubstrates 58 which cause the growth of polycrystalline, mercury.cadmium, telluride. in general, a class of substances used in practicingthe present invention comprises a solid material which is amorphous orwhich has a lattice structure at the growth temperature which promotesthe growth of dendrites or crystalite and which will remain chemicallyinert, i.e., constituents of the substrate will not react with thegrowth constituents to change the basic ternary compound composition.nor act as an impurity. The class of materials capable of meeting thesegeneral requirements is considered to be quite large; however, specificmaterials successfully used were sapphire (A1 quartz, hightemperatureglass such as Vycor and fine grained polycrystalline alumina. Othersubstances could also be used provided they remain solid at the growthtemperature. A further desirable condition for certain uses of thesensor is that the substrate be a nonconductive material althoughsuitable conductive or scmiconductive materials might be useful. a

As a preliminary to the growth of the polycrystalline HgCdTc, asubstrate is first precleaned in a vapor degreaser and acid etched.

In a specific example, a sapphire disk, approximately 1 cm. in diameterand 20 mils thick was selected, dcgreased in an ultrasonic cleaner andsubmerged in a potassium dichromate acid solution for several minutes.The size and thickness of the substrate is open to choice and depends tosome extent on the size and temperature capabilities of growthequipment. The thickness, for example, depends on the thermalconductivity properties of the substrate material, it being important inusing the growth apparatus of FIG. 1 that the cold finger and pedestalbe capable of reducing the temperature of the suhstrate on its growthsurface to the temperature required to promote polycrystalline ternarycompound formation. Prior to insertion of the sapphire substrate intothe growth chamber, the pedestal 56 and growth chamber 50 were cleanedfrom products of previous runs. The apparatus is then started up asdescribed in the said Manley et al. application, except that theback-etching operation is eliminated. Briefly, the procedure involvesplacing source materials in furnaces 10, ll, and I2, sealing the system,evacuating the system through valve 69, and then introducing hydrogen(or other inert carrier) gases from source 13 and vented through theapparatus to the atmosphere and ignited by coil 16. Vacuum pump is thenstopped and valve 69 closed. Furnaces l0, l2, l4, and 15 are turned onand gradually brought up to a desired temperature level. Initially Cdand Hg enriched streams will flow through furnace 14. Growth will notoccur, however. and mercury and cadmium will condense in the lowerportion of chamber 50. Lastly, the source furnace II is turned on tovolatilize tel|uri um from source 27 into hydrogen gas stream flowing inchamber 25. At the same time, cooling air is supplied to heat sink 60 todrop the temperature of the sapphire substrate 58 to the desired filmgrowing level. in a specific run using a sapphire substrate, a hydrogenflow rate of 60 cc./minute was used and the following operatingconditions were set:

Source furnace tcmpelaturc H] l Ctldlflluntl t'lti Source furnacetemperature I l llelltmuml i l 5 1' Source furnace tviopclulurt: I2trucrcuryl lilo t Mlung Iurnucc lentperulure 14 Hill Reaction furnacetemperature [5 Kilt! L Heat sink tcnipernlure on. L

With the above operating conditions, after a period ofapproxiniately 2hours, a polycrystalline material was produced on the surface of thesapphire substrate 58. At the completion of the run, the source furnacesl0 and ii and the mixing and reaction furnaces l4 and 15 are turned off.The source furnace 12 was kept on after the other furnaces were turnedoff to allow mercury vapor pressure to remain within prescribed levelsin chamber 50 to prevent mercury from being volatilized from the growthmaterial after the heat sink 60 is cut off. Source furnace l2 continuesto operate until reaction chamber 50 reaches a temperature of l00C. forHgCdTe and then shut off and opened to the atmosphere.

When inspected, the growth material was observed to have dcndriticcharacteristics with a random distribution over sub stantially theentire growth area. X-ray analysis of the growth material revealed thefollowing percentages of X-ray counts of the constituent elements:

Mercury 92 ill (mlrriium ZJl Tellurium 5 6| Sensors built with theabovedescribed growth material showed a wavelength sensitivity of l 1micron with a D-Star (500 K., L000 cps.) rating of 10.

A sensor element 71 using the polycrystalline growth, as shown in FIGS.2 and 3 comprises the growth substrate 58, the polycrystalline layer 72,and a pair of spaced thin film elec trodes 73 and 74. Wire leads and 76are bonded to the film electrodes 73 and 74 to permit electricalconnection to exter nal circuitry. The method for applying the gold filmelectrodes 73 and 74 comprised masking a strip of the polycrystallinelayer 72 using an inert wire or ribbon such as a nickel-chrome wire,then evaporating gold film onto the unmasked areas. The mask was thenphysically removed to expose the masked region. Following this, I milgold wires 75 and 76 were bonded to the film electrodes 73 and 74 bymeans of indium solder and a silver conducting epoxy. Other masking andbonding techniques could be employed and would readily occur to personsskilled in the art.

As previously stated, the substrate 58 is sapphire, quartz {monoorpolycrystalline) alumina, or Vycor. Since sapphire, quartz or Vycor aretransparent to IR radiation, the sensor device 71 can be used inapplications where layer 72 is exposed either directly to infraredradiation or indirectly through substrate 58. Devices of the type shownin FIGS. 2 and 3 were produced having properties described in the previous examples and table and in addition have demonstrated responsecharacteristics of 3 nanoseconds.

The apparatus for testing is shown in FIG. 4. in one type of test, thesensor device is placed in a cooling chamber 77. using liquid nitrogenas a coolant, and placed proximate a black body radiator 78 sourcehaving a temperature set precisely by temperature regulator 79 at 500"K. A radiation chopper, such as a rotating apertured disk 80, isoperated at a chopping rate of 500 and L000 c.p.s. The sensor device hasits leads 74 and 75 connected to a amplifier and biasing circuit 81having an output to a wave analyzer 82. Amplification and bias circuit81 and the wave analyzer 82 are well known. In specific tests, theamplifier and bias circuits 81 used was a Perry Mod 600 preamp and thewave analyzer was a HP 302A wave analyzer.

promoting polycrystalline growth on a surface of said substrate. 2. Aprocess for vapor growing ternary compounds in accordance with claim 1in which said Using the test setup shown broadly in FIG. 4, thewavelength 5 substrate is selected from a class of materials comprisingat a chopping rate of from I] to 250 c.p.s. measured 11 sapphire,quartz,heat resistant glass,alumina, or the like. microns peak response whileD-Star measured l". The test A pr ce for vapor growing ternary compoundsin acapparatus shown is for D-Star measurement. For wavelength cordancewith claim I in which said substrate IS sapphire. measurement, amonochrometer device, or the like, (not 4. A process for vapor growingternary compounds in acshown) is interposed between the chopper disk 80and detecl0 Cordance with claim 1 in which said substrate is quartz. tor7|. in using the monochrometer it may become necessary 5. A process forvapor growing ternary compounds in ac to increase the temperature of theblack body to compensate cofdance with fllaim 1 which $8111 Subslfale i'P Q for energy losses in the monochrometer. 6. A process for vaporgrowing ternary compounds in ac- Ozher examples of Samples and processConditions as we" cordance Wtllt claim 4 in which said substrate isamorphous as results, are set forth in the following table: 1 quartz 7.A process for vapor growing ternary compounds In ac TABLE I Sourcefurnace Mixing Analysis, percent X-ray temperature, C. furnace Reactionfluorescense rounts Wart"- temper- [Ieat sink furnace length, Samplenumber Subtratv Hg Cd Te attire, C. tempf temp.,(1. Ilg Cd Te micronsD-Star CKlufi Sapphire 230 370 5x5 850 603 300 91.6 2. 6 5.8 11 B CK83.d 300 370 515 850 690 800 (V2. 6 3. 0 4. 4 l0 l0 CK93. 280 370 516 350filb 800 ill. 7 2. El 5. 4 10 10 f CK80 285 370 I516 850 690 500 91.43,8 4. 7 6 10 CKBb Quartz 270 370 515 am 585 s00 39 5.4 5.6 s 10'lamorph.) CKlll Alumina... 280 370 515 850 600 500 97.8 1.5 5.7 12 l0 6CKfil Vycnr. 280 370 515 850 683 800 88.8 5.9 5.3 s 10= in the abovesamples, the substrates dimensions were within cordance with claim 1 inwhich said substrate has a lattice the range of l-2 cm. diameter and -40mils thickness. structure at growth temperature which causespolycrystalline Growth times were nominally 2 hours with a 60 cc./minutedeposition ofsaid elements. flow rate through each of the sourcefurnaces l0, l1, and I2. 8. A method for making an infrared sensordevice compris- The polycrystalline films were produced in sizes from 1to 2 ing: square centimeters. depositing polycrystalline mercury cadmiumtelluride com- While the above examples show growth process using apound ontoasubstrate; and specific apparatus and process with a specificcarrier gas, forming electrodes on said polycrystalline material in aother apparatus and process and materials might be employed. mannerwhich leaves at least a portion of said material ex- While the inventionhas been particularly shown and posed to infrared radiation. describedwith reference to preferred embodiments thereof, it 9. A method formaking an infrared sensor device in ac will be understood by thoseskilled in the art that the foregoing cordance with claim 8 in whichsaid polycrystalline mercury and other changes in form and details maybe made therein cadmium telluride is deposited on a sapphire substrateand without departing from the spirit and scope of the invention. saidelectrodes are thin film gold deposited on selected regions We claim: ofthe surface of said mercury cadmium telluride. l. A process for vaporgrowing ternary compound materials 10. A method for making an infraredsensor device in accomprised of mercury cadmium and telluriumcomprising: cordance with claim 8 in which said substrate is quartz andforming a gaseous mixture ofthe vapors of said elements in saidelectrodes are thin film gold layers deposited on said sura chamber;face of said mercury cadmium telluridc. maintaining said gaseous mixtureat a temperature which 1]. A method for making an infrared sensor devicein acprevents binary combinations ofsaid elements; cordance with claim 8in which said substrate is vycor. rapidly supersaturating said mixtureand condensing said elements onto a solid substrate havingcharacteristics for S0 qg gg UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION 3.642,29 Dated Febrggry l5 1972 Robert E. Lee, Philip S.McDermott Inventofla) and Edward S. Pan

Patent No.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1', after the title (line 1) and before "Cross-References ToRelated Applications" (line 2) insert the paragraph The invention hereindescribed was made in the course of, or under a contract, or subcontractthereunder, with the Department of Defense.-

Signed and sealed this 11 th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOT'I'SCHALK Attesting Officer Commissionerof Patents

2. A process for vapor growing ternary compounds in accordance withclaim 1 in which said substrate is selected from a class of materialscomprising sapphire, quartz, heat resistant glass, alumina, or the like.3. A process for vapor growing ternary compounds in accordance withclaim 1 in which said substrate is sapphire.
 4. A process for vaporgrowing ternary compounds in accordance with claim 1 in which saidsubstrate is quartz.
 5. A process for vapor growing ternary compounds inaccordance with claim 1 in which said substrate is amorphous.
 6. Aprocess for vapor growing ternary compounds in accordance with claim 4in which said substrate is amorphous quartz.
 7. A process for vaporgrowing ternary compounds in accordance with claim 1 in which saidsubstrate has a lattice structure at growth temperature which causespolycrystalline deposition of said elements.
 8. A method for making aninfrared sensor device comprising: depositing polycrystalline mercurycadmium telluride compound onto a substrate; and forming electrodes onsaid polycrystalline material in a manner which leaves at least aportion of said material exposed to infrared radiation.
 9. A method formaking an infrared sensor device in accordance with claim 8 in whichsaid polycrystalline mercury cadmium telluride is deposited on asapphire substrate and said electrodes are thin film gold deposited onselected regions of the surface of said mercury cadmium telluride.
 10. Amethod for making an infrared sensor device in accordance with claim 8in which said substrate is quartz and said electrodes are thin film goldlayers deposited on said surface of said mercury cadmium telluride. 11.A method for making an infrared sensor device in accordance with claim 8in which said substrate is vycor.